The invention relates to electroerosion printing and to recording materials characterized by an improved hydrophilic, conductive or resistive lubricant topcoat, especially for use in the production of direct offset masters.
Electroerosion printing is a well-known technique for producing markings, such as, letters, numbers, symbols, patterns, such as, circuit patterns, or other legible or coded indicia on recording material in response to an electric signal which removes or erodes material from the surface of the recording material as the result of spark initiation.
The surface which is eroded or removed to provide such indicia on the recording material is usually a thin film of conductive material which is vaporized in response to localized heating associated with sparking (arcing) initiated by applying an electric current to an electrode in contact with the surface of a recording material comprising the thin conductive film on a flexible nonconductive backing or support. In the present state of the technology the thin conductive film is usually a thin film of vaporizable metal, such as, aluminum.
Electroerosion printing is effected by the movement of a stylus or a plurality of styli relative to the surface of specially prepared recording media. Electrical writing signals are fed to the stylus to provide controlled electrical pulses which generate sparks at the surface of the recording material to selectively heat and remove by evaporation a layer of the recording material; the locations from which material is removed correspond to the indicia or images which are to be recorded.
In the course of this process, the stylus is moved relatively to a surface of the recording material and in contact with the removable layer, e.g., a thin film of vaporizable material, usually a metal, such as aluminum.
Due to the fragility of the thin conductive layer and stylus pressure, considerable scratching (undesired removal of the removable layer) is observed to take place during electroerosion printing.
It has been recognized for some time, therefore, that the use of a lubricant and/or protective overcoat on the surface of such electroerosion recording materials would be helpful to reduce scratching by the stylus. After some investigation, lubricants comprising long chain fatty acids were adopted. Even with the use of such lubricants, however, some stylus scratching of the thin aluminum film of electroerosion recording materials continues to be observed. Therefore, efforts continued to be directed to finding a superior lubricant--protective layer composition for the surface of electroerosion recording materials.
In co-pending application Ser. No. 454,744 filed Dec. 30, 1982, entitled "Graphite Lubricant in Electroerosion Printing of Direct Offset Photonegative" and filed in the name of Mitchell S. Cohen, one of the co-inventors herein, the entire disclosure of which is incorporated herein by reference, there is described an improved electroerosion recording material having an overlayer of a protective lubricant composition comprising conductive particles of high lubricity dispersed in a polymeric binder on the stylus-contacting surface of the material. While various laminar solids and other soft compounds and soft metal particles are disclosed in Ser. No. 454,744 for use as the lubricant, conductive particles, graphite, due to its cost, effectiveness and easy dispersibility is the preferred material described therein.
When an electroerosion recording material is to be used as a direct offset master for printing with oleophilic inks, it is necessary to have hydrophobic-hydrophilic mapping of the image and non-image areas, respectively. With the electroerosion material of Ser. No. 454,744, after electroerosion printing, it is necessary to completely remove remaining topcoat to expose the hydrophilic surface of the conductive metal in the non-image areas in order to obtain the necessary differential in wetting characteristics. Either the support, such as Mylar.RTM., or an intermediate layer of hydrophobic, hard, tackfree coating such as a coating of an organic polymer-silica dispersion, provides the hydrophobic, ink-receptive image areas after electroerosion recording. The removal of a lubricant topcoat in the preparation of an offset master is undesired as it presents an extra process step, usually requires use of an organic solvent which could affect or alter the other layers of the recording material and in general increases the chance of damage and/or delamination of the aluminum layer. Thus, it would be advantageous to provide an abrasion-resistant recording medium which does not require removal of the topcoat after electroerosion printing for use as an offset master.
U.S. Pat. No. 3,509,088 to Dalton describes electrical-signal-responsive films containing dispersed carbon black particles. In one embodiment, the carbon black particles are mono-layer coated with an adsorbate and then admixed with a multi-phase resin complex to form a film which can be applied over a conducting film.
U.S. Pat. No. 4,317,123 to Namiki et al is directed to a thermal recording material including a protective layer formed of film forming high molecular weight materials such as cellulose or derivatives thereof, etc. and which can contain various pigments and matting agents such as carbon black, colloidal silica, etc.
Among prior disclosures relevant to electroerosion printing, U.S. Pat. No. 2,983,220, Dalton et al, discloses a lithographic coating on an electroerosion recording sheet; the coating may be a copolymer binder system containing zinc oxide and zinc sulfide. An internal layer containing conductive material, such as graphite, is disclosed in U.S. Pat. No. 3,048,515, Dalton.
An electroresponsive recording blank having a removable masking layer containing a luminescent material is described in U.S. Pat. No. 2,554,017, Dalton. Other prior art providing further general background in the field of electroerosion printing includes U.S. Pat. Nos. 3,138,547, Clark and 3,411,948, Reis. High temperature lubricants comprising graphite in oil are also known, as is described in U.S. Pat. No. 3,242,075, Hunter.